[PATCH] I2C: fix sx200_acb build on other arches

scx200_acb is only useful on a few Geode-based systems, and won't
compile on non-x86 systems due to the lack of asm/msr.h, as reported
by Andrew Morton. Thus, we should make that driver depend on X86.

Signed-off-by: Jean Delvare <khali@linux-fr.org>